1. Field of the Invention
This invention pertains generally to driver circuits, and more particularly to light emitting diode (LED) driver circuits which provide electrical conversion and isolation from supply mains.
2. Description of Related Art
There is significant impetus for shifting toward light emitting diode (LED) lamps within existing fixtures, such as screw-in Edison-bulb type lamps. Residential LED lighting, for example, is an emerging market with several manufacturers producing screw-in retrofit LED lamps for use in standard 120 Vrms Edison sockets. Perhaps the greatest share of the overall demand for replacement LED lamp devices is in devices directed at the power range of from 5 to 15 watts.
Space, efficiency and longevity are concerns within these LED lamp devices. In many LED lamps, the overall space available for circuitry may be only approximately 1×1×2.5 inches, with specific dimensions depending upon the particular lamp envelope package. Efficiency considerations are important in view of both providing the highest lumen output for a given power input, and in regard to the necessity of dissipating heat from power that is not converted to light. Nominal lifetimes for these lamps are targeted in the range of up to 50,000 hours.
As LED elements can not be driven directly from an AC line, such as 110 VAC, a conversion circuit is required within the packaging of each lamp device. The conversion circuit should nominally exhibit high electrical conversion efficiency (−90%), with high power factor (meeting the IEC 61000-3-2, part C specification on harmonics), while providing regulated current to a single series string of LED devices.
These LED drive circuits may also require bulk electrolytic capacitance, that is usually considered the weak link in terms of lifetime and reliability of the devices, while also taking up significant circuit space.
Among the many unique challenges to creating lamps for this market, thermal management is perhaps paramount. Unlike incandescent lamps, which operate properly at filament temperatures up to 2500° C., LED junctions are limited to far cooler temperatures, typically less than 100° C. The heat produced by an LED is not directly convected from the front of the LED chip, but instead must be conducted through the back-side of the chip. It is critical to minimize the total thermal resistance from junction to ambient air toward providing adequate cooling of the LED.
It would seem ideal to directly bond the LED devices to a large metal heat sink exposed to the ambient-air to minimize total thermal resistance. Although, in this configuration, the heat sink becomes a safety hazard because many existing fixtures, such as standard Edison sockets, lack an earth (ground) connection. Replacing a metal heat sink with a non-electrically conductive one, such as ceramic, results in significantly lower levels of thermal conductivity. For example, currently proposed ceramics provide thermal conductivity that is still an order of magnitude less than that of common metals, e.g., 24 W/m-K for Rubalit versus 210 W/m-K for aluminum. One solution toward overcoming the isolation problem, when using large metal heat sinks, is to galvanically isolate the LEDs from the AC mains utilizing a transformer. It will be appreciated that galvanic Isolation involves forced isolation between two circuits so that no metal conduction path exists between those circuits. However, using these isolation transformers at the low power line frequencies involved significantly increases circuit size and cost when applied to LED driver circuits.